While a traditional concern of any logging operation is the efficient transportation of felled timber from a forest to processing plants, modern logging planners are also concerned with minimizing safety hazards and environmental damage resulting from such operations. Thus, while clear-cutting timber may be the most "efficient" way to log a forest, logging planners may opt to selectively harvest timber because of environmental or timber management concerns.
It is important, therefore, that modern logging equipment be designed with the broadened concerns of logging planners in mind. Ideally, logging equipment will be adaptable for use in either clear-cutting or selective harvesting situations. Additionally, it is desirable to use logging equipment which will minimize the disruption of the soil in the area being logged. Such disruption can often result in excessive soil erosion, which will not only be detrimental to the forest land but can adversely impact aquatic life forms in nearby streams.
After timber is harvested, the resulting logs are transported to a landing. A landing is a generally level area, situated near a logging road, from which logs are loaded on trucks and hauled to processing plants. The act or process of conveying logs to a landing is known as "yarding." When timber is harvested on grades of less than 25-35%, tractors or other heavy equipment, such as skidders, may be used for yarding. Such equipment is generally efficient only at haul distances below 1,000 feet and works best in clear-cutting operations. When harvesting steeper slopes or hauling over longer distances, some type of cable yarding system is often employed.
One such system is a skyline system. In a skyline system, a cable known as a skyline is stretched taut between two spars to extend over sloped terrain. A carriage equipped with grooved wheels rides on the skyline to carry logs to a landing positioned near one of the spars. A second cable, known as the mainline, extends from the uphill spar to the carriage. The mainline is reeled in to pull the carriage uphill and paid out as the carriage moves downhill due to gravity.
To operate a skyline system, the carriage is lowered to a desired location on the skyline and secured in place. Chokers or grapple hooks are lowered from the carriage and attached to nearby logs. Once the logs are attached to the chokers or grapple hooks, they are raised up to the carriage and the carriage is moved either uphill or downwhill to a landing, where the logs are lowered and released.
The skyline is usually elevated at at least one location. When logging a concave slope, for example, the uphill spar is normally elevated by a portable tower, while the downwhill spar is secured to a tree trunk or the like, as shown in FIG. 1. Elevating the skyline allows the logs to be transported to the landing without dragging them on the ground. This procedure makes it easier to pass over ground obstacles and lessens environmental damage by minimizing soil disruption caused by dragging the logs over the ground.
An important characteristic of a skyline system is its lateral reach, or yarding width. To set up a skyline system for operation, a corridor beneath the skyline must be clear-cut to create a passageway to transport logs to the landing. Areas on either side of this skyline corridor can be selectively harvested, however. If the lateral reach of the choker or grapple hooks is too short to cover a given area, one or both spars must be relocated and a new skyline corridor cut, thus lessening the percentage of the area which can be selectively harvested. Setting up additional spar locations is also time-consuming and inefficient, even if the skyline system is being used to clear-cut an entire area.
Thus, a skyline system which will function well for both clear-cutting and selective harvesting must have adequate lateral reach. Existing skyline systems use a variety of cable arrangements to rig the chokers to provide adequate lateral reach. Some systems, for example, include a spool of cable mounted inside the carriage which is reeled in or paid out to raise or lower the chokers. The spool may be coupled to a pair of mainlines for rotation.
Another arrangement involves passing the mainline through the carriage and using it to raise and lower the logs. When this arrangement is used, some sort of slack pulling device is usually needed to ensure that when the mainline is paid out at the uphill spar, it will extend through the carriage and lower the chokers rather than merely drooping between the uphill spar and the carriage. One type of slack pulling device is a driven mainline sheave, which is positioned on the carriage and will help pay out the mainline. These mainline sheaves can be driven by hydraulic motors on the carrier itself. Such slack puller systems often include a pressure roller positioned adjacent the main line sheave to force the main line against the main line sheave and thereby aid in paying out the main line through the carriage. Conventional pressure rollers are freely rotating devices, and even with the aid of such pressure rollers, the main line may sometimes droop between the carriage and the spar rather than paying out properly.
For a skyline system to operate properly, it is necessary to have a safe and reliable method of stopping the carriage at a desired location along the skyline so that logs can be picked up and transported to the landing. One existing method is to use a "stopper" to prevent movement of the carriage beyond a desired point. A "stopper" is a device which rides along the skyline and may be manually clamped at any location along the skyline to prevent passage of the carriage beyond such location.
Another method of stopping the carriage is by means of a clamp which is positioned on the carriage itself. Such clamps may be hydraulically actuated, such as the one disclosed in U.S. Pat. No. 4,164,289. In any hydraulically actuated clamp, it is important to have some sort of backup system so that the carriage will not begin to move along the skyline if the hydraulic pressure in the carriage system drops. Such movement can be particularly dangerous to workers rigging the logs.
Hydraulically controlled components, such as a skyline clamp or a mainline sheave, are particularly advantageous because they can be operated by radio control. Radio-controlled devices often reduce the manpower requirements of a logging crew as well as eliminate the need for additional equipment, such as cables running to the carriage or separate stoppers to stop the carriage. Additionally, radio-controlled devices allow the log riggers to quickly and accurately control the carriage functions. Such an arrangement is not only more efficient, but will be much safer as well, as a rigging crew need not signal a distant operator to halt carriage operations in case of an emergency. In order to take advantage of all the hydraulic and remote control possibilities on a skyline system, there must be a means to provide adequate pressure to a hydraulic system within the carriage. Existing systems use a single pump coupled to the skyline wheels to pressurize the carriage hydraulic system. As the carriage moves up and down the skyline, the skyline wheels rotate. By coupling this rotation to the pump, the carriage hydraulic system can be pressurized. If the pump selected is too small, however, inadequate system pressure will be developed to operate the hydraulic components. On the other hand, if the pump is too large, it will resist rotation of the skyline wheels and they will merely skid down the skyline without rotating when the carriage is not loaded with logs. Under these conditions, the carriage hydraulic system is only being pressurized when the carriage is moving uphill with a load of logs. Thus, pump size is currently something of a compromise between choosing a pump which will be small enough to operate under all conditions and choosing one which will be large enough to take advantage of the full pumping capacity of a loaded carriage.
It can be seen, therefore, that there is a need for a skyline carriage with a hydraulic system with sufficient pressurizing capacity to operate a plurality of carriage components, such as clamps and slack pullers. Additionally, there is a need to provide hydraulically actuated components which will function safely should hydraulic pressure be lost. There is also a need for a hydraulically actuated slack pulling mechanism which will adequately eliminate drooping problems.